Complex electronic apparatus may require the interconnection of thousands of individual electronic devices. To manage the large number of required interconnections, such apparatus is often configured by affixing devices to a circuit boards (circuit packs), and by interconnecting these circuit packs via one or more printed wire backplanes. Collections of interconnected circuit packs are often contained in housings referred to as equipment shelves.
As circuit and power densities increase, heat dissipation presents a significant challenge. Increasingly, forced air convective cooling is used in equipment shelves to provide the required cooling capacity. In a typical shelf configuration, circuit packs are positioned vertically with sufficient spacing to allow for airflow between adjacent packs. Airflow is generated by fans located above or below the shelf, and directed past the packs by enclosing the sides, rear and front of the shelf to be relatively airtight. Air flowing past the boards carries heat away from the electronic devices on the boards, and is then exhausted from the shelf. Ambient air is drawn into the shelf by the fans to refresh and continue the airflow.
The effectiveness of this approach can be impacted by imbalances in the equipment shelf. For example, to allow for future increases in system capacity, fewer than all circuit pack positions may be equipped. Fan assemblies for shelves that are partially equipped may generate uneven airflow backpressures across the equipped and unequipped positions.
In an equipped position, electronic devices are present in the airflow path, and these devices create resistance and an associated pressure drop over the airflow path. In an unequipped position, no electronic devices sit in the airflow path, and pressure differentials over the path are substantially less. Because air moving through the shelf seeks a path of least resistance, the presence of unequipped positions leads to an increased airflow through the unequipped positions and a decreased airflow through the equipped positions. While fans with increased capacity can substantially overcome this problem, they may also generate increased noise, require added space, consume additional power, generate additional cost, and experience reduced life. In addition, fan failures may occur, increasing the risks of overheating and reducing the life of the electronic devices.